In recent years, metal-on-carrier, which is formed of a metal carried on a carrier such as carbon material, ceramic/metal oxide material, metallic material, or organic polymer material, has become of interest as functional material in a variety of fields, and extensive research and development has been conducted on new applications of such a metal-on-carrier.
Examples of such metal-on-carriers which have heretofore been developed include (1) electrically conductive particles formed of insulating particles (e.g., resin particles) whose surfaces are coated with a metal; (2) a catalyst for decomposing a disinfectant or bleaching agent, the catalyst being formed of a resin material having an electrical-conductivity-imparted surface on which a noble metal is carried; (3) a catalyst for purification of automobile exhaust gas, the catalyst being formed of a porous support on which a noble metal is carried; and (4) a photocatalytic thin film formed of a high-catalytic-performance thin film coated with a noble metal. In the field of fuel cell, developed applications of such a metal-on-carrier include (5) a reforming catalyst for reforming a hydrocarbon compound or an oxygen-containing hydrocarbon compound, thereby generating hydrogen, the catalyst being formed of an inorganic oxide support on which a noble metal (e.g., ruthenium) is carried; (6) a shift reaction catalyst for reducing the amount of carbon monoxide contained in hydrogen gas, the catalyst being formed of an inorganic oxide support on which a noble metal is carried; and (7) an electrode catalyst for a fuel cell, the catalyst being formed of a carbon material on which a noble metal is carried.
General features and background of the aforementioned metal-on-carriers will next be described.
[Electrically Conductive Particles]
An electrode section of a liquid crystal device or the like employs an anisotropic electrically conductive member which conducts electricity between specific electrodes or in a specific direction through deformation of electrically conductive particles under application of pressure. Such an anisotropic electrically conductive member incorporates electrically conductive particles formed of insulating particles (e.g., resin particles) whose surfaces are coated with, for example, gold. In general, an inductor or multi-layer capacitor employed in electronic parts is produced by laminating a magnetic layer on an electrically conductive layer, and subjecting the resultant laminate to sintering. Generally, such an electrically conductive layer is formed from a conductive-material-forming paste containing electrically conductive particles.
[Disinfectant-decomposing Catalyst]
Peroxides such as hydrogen peroxide and ozone are useful substances having disinfectant, antiseptic, and bleaching effects. However, a large amount of such a peroxide may be harmful to (i.e., adversely affect) the human body. Therefore, after intended effects have been attained by such a peroxide employed in a large amount, the thus-employed peroxide is subjected to neutralization-decomposition treatment. As has been known, a noble metal (e.g., platinum) serves as a decomposition catalyst in such decomposition treatment. For example, lightweight catalyst materials of different structures for decomposing a disinfectant, a bleaching agent, or the like can be produced by, for example, imparting electrical conductivity to the surface of a resin material formed of, for example, polyphenylene ether (hereinafter may be abbreviated as “PPE”) or polyphenylene sulfide (hereinafter may be abbreviated as “PPS”) in advance, and by causing a noble metal material to be carried on the resin material.
[Catalyst for Purification of Automobile Exhaust Gas]
In recent years, Nox occlusion-reduction-type catalysts have been widely employed as catalysts for purification of lean-burn automobile exhaust gas. Such an NOx occlusion reduction-type catalyst includes particles of a noble metal having catalytic activity (e.g., platinum or palladium), and a carbonate of a metal (generally an alkaline earth metal such as barium), the particles and the carbonate being carried on a porous support formed of pellets of ceramic material (e.g., alumina or zirconia) or a honeycomb molded material of such ceramic material, or formed of a metallic honeycomb structure coated with ceramic material. In such an NOx occlusion-reduction-type catalyst, noble metal particles serve as a catalytic component for promoting decomposition of NOx, and an alkaline earth metal serves as an NOx-occluding agent.
[Photocatalytic Thin Film]
When a photocatalytic material (hereinafter may be referred to simply as a “photocatalyst”) is irradiated with light having an energy equal to or greater than the band gap energy thereof, electrons are excited to the conduction band, and holes are provided in the valence band. The thus-excited electrons reduce oxygen on the surface of the photocatalyst, to thereby form superoxide anions (.O2−), whereas the thus-generated holes oxidize hydroxyl groups on the photocatalyst surface, to thereby form hydroxyl radicals (.OH). As has been known, the thus-formed reactive oxygen species exhibit strong oxidation/decomposition performance, and thus are highly efficient in decomposing organic substances deposited onto the surface of the photocatalyst.
Titanium dioxide (in particular, anatase titanium dioxide) is practically useful as such a photocatalytic material. For the purpose of promoting the photocatalytic activity of a layer formed of such a photocatalytic material, the layer is provided with a coating layer formed of a platinum group metal (e.g., platinum, palladium, rhodium, or ruthenium).
[Metal-on-carriers in the Field of Fuel Cell]
In a fuel cell, chemical energy is converted into electrical energy through electrochemical reaction between hydrogen and oxygen. Fuel cells, which are characterized by high energy-utilization efficiency, have been extensively studied for practical use; for example, consumer use, industrial use, or automotive use.
Studies have been conducted on employment, as a hydrogen source, of methanol, liquefied natural gas predominantly containing methane, city gas predominantly containing such natural gas, a synthetic liquid fuel formed from natural gas, a hydrocarbon fuel such as petroleum hydrocarbon (e.g., LPG, naphtha, or kerosene), or an oxygen-containing hydrocarbon fuel.
In the case where hydrogen is generated by use of the aforementioned hydrocarbon fuel or oxygen-containing hydrocarbon fuel, reforming treatment (e.g., steam reforming or partial oxidation reforming) is carried out. In such a case, a catalyst formed of an inorganic oxide support on which a noble metal (e.g., ruthenium) is carried is generally employed as a reforming catalyst.
Generally, hydrogen gas obtained through the aforementioned reforming treatment contains CO. In a fuel cell; in particular, a low-temperature operation fuel cell (e.g., a polymer electrolyte fuel cell), CO is prone to poison a platinum catalyst serving as an electrode. Therefore, CO concentration must be reduced through conversion of CO into a nontoxic substance (e.g., CO2). Such CO reduction is generally carried out through a method employing shift reaction. Such shift reaction employs a catalyst formed of an inorganic oxide support on which a noble metal is carried.
From the viewpoint of promotion of chemical reaction, an electrode constituting a polymer electrolyte fuel cell is formed of a carbon material (e.g., graphite or carbon black) on which a noble metal (e.g., platinum) is carried.
Such a noble-metal-on-carrier is formed through a physical vapor deposition (PVD) method (e.g., vacuum deposition or sputtering) or a wet method (e.g., electroplating, electroless plating, or a method for causing colloidal metal particles to be carried on a carrier (hereinafter the method may be referred to simply as a “colloidal metal carrying method”)). The noble-metal-on-carrier formation method may be appropriately selected from among the aforementioned methods in accordance with, for example, the use of the metal-on-carrier or the type of a carrier to be employed. Of these methods, a colloidal metal carrying method—in which a metal nanocolloidal liquid containing nanocolloidal metal particles is applied to a carrier through a technique (e.g., immersion, spraying, or evaporation to dryness), to thereby cause the nanocolloidal metal particles to be carried on the carrier—is advantageous in that, for example, the method is easy to perform, and does not require any expensive coating apparatus. As used herein, the term “nanocolloidal particles” refers to colloidal particles having a particle size of less than about 100 nm.
However, in such a metal nanocolloidal liquid, generally, nanocolloidal metal particles exhibit poor dispersion stability, and are prone to form aggregates. Therefore, generally, a water-soluble polymer compound (e.g., polyvinyl alcohol, polyvinyl pyrrolidone, or gelatin) or a protective colloid-forming agent (e.g., a surfactant) is added to the metal nanocolloidal liquid, and a protective colloid is formed, whereby the dispersion stability of the nanocolloidal metal particles is improved.
For example, there have been disclosed a method in which a metal oxide thin film deposited on an insulating substrate is immersed in a noble metal colloid obtained by adding, to an aqueous noble metal chloride solution, an aqueous polyethylene glycol monooleyl ether solution serving as a protective colloid-forming agent, to thereby cause the noble metal to be carried on the metal oxide thin film (see, for example, Patent Document 1); a method for producing an exhaust gas purifying catalyst, in which a noble metal colloidal liquid is prepared by use of a quaternary ammonium salt having at least one C1-C4 alkyl group serving as a protective colloid-forming agent, and the colloidal liquid is adsorbed onto a porous support (see, for example, Patent Document 2); and a method for producing a photocatalytic thin film carrying fine noble metal particles, in which a noble metal fine particulate colloid which has been stabilized with a surfactant is applied onto a thin film having photocatalytic function, followed by thermal treatment at about 400 to about 600° C. in a reducing atmosphere (see, for example, Patent Document 3).
However, in the case where such a protective colloid-forming agent is employed, when nanocolloidal metal particles are caused to be carried on a carrier, the protective colloid-forming agent is deposited on the surfaces of the nanocolloidal metal particles carried on the carrier; i.e., the resultant metal-on-carrier contains an organic substance. In some cases, such an organic-substance-containing metal-on-carrier may fail to sufficiently perform its intended function. In such a case, the metal-on-carrier must be subjected to treatment for removal of the organic substance (e.g., firing treatment). However, in some cases, the carrier cannot be subjected to firing treatment. Thus, employment of a protective colloid-forming agent also poses a problem in that a limitation is imposed on the type of the carrier to be employed.
Known methods for producing a metal nanocolloidal liquid employing no protective colloid-forming agent include a method in which a reducing agent is added to a metal chloride solution, and fine metal particles are formed through reduction of metallic ions (see, for example, Patent Document 4 and Non-Patent Document 1).
When colloidal particles are caused to be carried on a carrier by use of such a metal nanocolloidal liquid containing no protective colloid-forming agent, generally, there is employed a method for causing the colloidal particles to be carried on the carrier through spontaneous adsorption. However, this method raises problems in that the particles are caused to be carried on the carrier at low rate, and the amount of the particles to be carried cannot be increased to a sufficient extent.
When a metal-on-carrier is to be produced by use of a metal nanocolloidal liquid, desirably, a maximum possible amount of nanocolloidal metal particles is caused to be carried on a carrier in one operation, from the viewpoint of operation efficiency. Therefore, a metal nanocolloidal liquid to be employed is required to contain colloidal particles in a large amount.
However, the aforementioned metal nanocolloidal liquid production method employing no protective colloid-forming agent involves a problem in that when a metal nanocolloidal liquid containing nanocolloidal metal particles in a large amount is to be prepared, the colloidal particles are prone to aggregate and precipitate. Conceivably, when the distance between fine metal particles becomes excessively small, such particle aggregation occurs as a result of electrostatic shielding and insufficient repulsion between the metal particles, since merely ions adsorbed onto the metal particles contribute to dispersion of the particles.
Therefore, demand has arisen for a metal nanocolloidal liquid containing nanocolloidal metal particles in a large amount, containing no protective colloid-forming agent, and exhibiting good dispersion stability. In a conventional technique, when a metal nanocolloidal solution is to be prepared from platinum particles without employing a protective colloid-forming agent, the amount of the particles contained in the solution is limited to about 150 mass ppm.
Conventionally, spraying has been widely employed as a technique for causing nanocolloidal metal particles to be carried on a carrier by use of a metal nanocolloidal liquid. However, a conventional spraying technique may pose problems in terms of, for example, safety of volatile components, regulation of the concentration of a spraying liquid, damage to a carrier, and safety to the human body. In addition, such a spraying technique involves a problem in that, for example, a treatment (e.g., firing or reduction) for removal of a protective colloid-forming agent may be required after nanocolloidal metal particles have been caused to be carried on a carrier.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2000-87248
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-1119
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 11-71137
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2001-224969
Non-Patent Document 1: “Surface,” Vol. 21, No. 8, pp. 450-456 (1983)